A plasma arc torch is generally utilized for metal workings including cutting, welding, surface treating, dissolving and annealing Workings. As such plasma arc torch, one for the metal cutting working generally has a structure, for example, shown in FIG. 1.
In FIG. 1, reference numeral 13 denotes a torch body, reference numeral 1a is an electrode holder supported by the torch body 13, and reference numeral 1 is an electrode member embedded in and and joined to the electrode holder 1a. Reference numeral 2 is a nozzle supported by the torch body 13 through a nozzle supporting member 3 so as to surround the electrode member 1 and be positioned on a front end side of the electrode member 1, reference numeral 4 is a nozzle cap supported by the torch body 13, surrounding the nozzle 2 except the front end portion thereof and having a front end secured to the front end portion of the nozzle 2, and reference numeral 5 is a nozzle protecting cap supported by the torch body 13 and surrounding the outer side of the nozzle cap 4.
A plasma gas passage 6 is formed to a peripheral portion of the electrode member 1 so as to communicate with the nozzle 2 from this peripheral portion, and a cooling water passage 7 is further formed between the nozzle 2 and the nozzle cap 4. Still furthermore, a secondary gas passage 8 is formed between the nozzle cap 4 and the nozzle protecting cap 5 so as to open to the front end side of the nozzle 2.
The nozzle protecting cap 5 is in an electrically insulated from the nozzle cap 4.
The electrode body 1a is formed with a cooling water chamber 9 for cooling the electrode member 1, and the cooling water chamber 9 communicates with the cooling water passage 7. The cooling water chamber 9 is connected to a cooling water flow-in passage 10 and the cooling water passage 7 is connected to a cooling water flow-out passage 10a. On the other hand, a plasma gas flow-in passage 11 is connected to the plasma gas passage 6 and a secondary gas flow-in passage 12 is also connected to the secondary gas passage 8.
The torch body 13 serves to support the respective members and portions mentioned above and is electrically insulated from the electrode member 1 and the nozzle 2, and the nozzle protecting cap 5 is screw engaged with the torch body 13.
The electrode member 1 used for such plasma arc torch is formed of, in consideration of durability under a high temperature condition, a heat resistant material of homologous element such as hafnium (Hf), zirconium (Zr), titanium (Ti), etc., and the electrode member 1 is joined to the electrode body 1a formed of copper (Cu) by means of brazing.
The joining of the electrode member 1 to the electrode body 1a will be performed, by a method other than the brazing, as disclosed in the Japanese Patent Publication No. HEI 5-70250 teaching a technique that a sleeve made of such as silver is inserted into the electrode body 1a and the electrode member 1 is then inserted into the sleeve and fixed thereto. In this structure, however, surfaces to be joined of both the electrode member 1 and electrode body 1a have irregularity, which will constitute heat conduction resistance, thus being inconvenient.
In order to obviate this defect, when the electrode member 1 and the electrode body 1a are joined by means of brazing, the irregular surfaces are embedded with a brazing material, providing an excellent heat conduction performance therebetween and providing an improved cooling effect even if the electrode member 1 is formed of hafnium having a bad heat conduction property.
According to such fact, as mentioned above, the electrode member 1 is joined to the electrode body 1a by means of brazing and a silver (Ag) material is used as a brazing material including copper (Cu) of from several % to several tens % to lower a melting point thereof.
FIG. 2 shows a condition of a joined portion in a case where the hafnium electrode body 1 is the copper electrode body 1a by using a brazing material 14 formed of a silver material including copper of 30% (Ag+30%Cu), and a mixed crystal layer 15 formed of Hf as a material of the electrode member and Cu contained in the brazing material 14 (Hf-Cu mixed crystal layer) is formed to a boundary surface between the electrode member 1 and the brazing material 14.
The Hf-Cu mixed crystal layer 15 is formed of an extremely hard material having a large brittleness. For example, in the measurement of the inventors, such Hf-Cu mixed crystal layer 15 has a Vickers hardness of about 500-600, and on the other hand, the Hf electrode member 1 and the Ag brazing member 14 have Vickers hardness of about 200 and 100, respectively.
In a case where an arc generation/extinction is repeated by using the electrode member 1 in such joined condition, a crack 16 is caused at the front end portion of the contact boundary between the Hf of the electrode member 1 and the Hf-Cu mixed crystal layer.
As a main reason of such generation of crack, thermal stress due to rapid temperature increasing at the arc generation time will be considered. When such crack 16 is caused at the joined surface of the electrode member 1, this portion exhibits a lower cooling effect, and consumption of the electrode material at this portion will rapidly progress, which results in an extremely lowered durability as the electrode member for the plasma arc torch.
Further, although the above example is one using hafnium for the electrode member 1, it has been confirmed that, in a case where the electrode member 1 is formed of zirconium or titanium and joined by means of brazing member containing copper, a mixed crystal layer composed of the respective materials of the electrode member and the brazing member has been also formed to a boundary surface therebetween. Such mixed crystal layer has a large hardness and large brittleness as like in the Hf-Cu mixed crystal layer 15 as mentioned above.
The present invention was conceived in view of the above fact, and in consideration of the above problems, the basic reason therefor resides in that the copper mixed crystal layer having a large brittleness is formed to the portion contacting the electrode member, and accordingly, the present invention aims to provide an electrode for a plasma arc torch capable of extremely improving durability with no generation of crack to a peripheral portion of an electrode member even by repeatedly performing an arc generation/extinction process under a condition that no copper mixed crystal layer constituting a brittleness layer is formed between the electrode member and a brazing material.